The transmission capacity of single-core optical fibers used in the present situations is approaching its limit due to the recent rapid traffic increase in optical communications. As a means for further expanding the communication capacity, a multi-core fiber, which is a fiber having a plurality of cores formed therein, has been proposed.
In order to realize a transmission line by a multi-core fiber, an optical-fiber amplifier adapted to a multi-core fiber is required. An optical fiber doped by a rare-earth material such as erbium is used for the optical-fiber amplifier. Entering pump light into such a rare-earth-doped optical fiber allows light to be amplified.
As for an optical-fiber amplifier, there are core excitation, in which pump light enters into cores, and cladding excitation, in which pump light enters into claddings. Here, if cladding excitation is chosen for the multi-core fiber, light sources for pump light as many as the cores become unnecessary so to suppress the electrical power consumption. Therefore, a method for entering pump light into claddings of a multi-core fiber configuring an optical-fiber amplifier is required.
As such an optical-fiber amplifier, provided is an optical-fiber collective amplifier, in which a plurality of cores are positioned in a cladding and pump light is introduced into a multi-core fiber having a cladding that is doped with an optical amplifying material (Patent Document 1).